Senator John McCain, chairman of the Senate Armed Services Committee, said on Thursday that he may consider issuing a subpoena because the White House has not been forthcoming with details of an ambush in Niger in which four U.S. soldiers were killed.

The attack earlier this month, which U.S. officials suspect was carried out by a local Islamic State affiliate, has thrown a spotlight on the U.S. counter terrorism mission in the West African country, which has about 800 U.S. troops.

The U.S. military is investigating the incident to find out what went wrong and what, if any, changes need to be made.

“It may require a subpoena,” McCain said when asked what steps his committee might need to take to determine what happened to the four troops.

Asked what information the committee still needed, McCain said “everything.” When questioned if the White House had been forthcoming with the information needed by the committee, he added, “of course not”

He said he had a good conversation with President Donald Trump’s national security adviser, H.R. McMaster, and hoped the White House would eventually provide the information needed by the committee.

From initial accounts, the 40-member patrol, which included a dozen U.S. troops, came under swift attack by militants riding in a dozen vehicles and on about 20 motorcycles.

The mission had been seen as a relatively lower-risk endeavor for America’s elite commandos and there was no armed air cover at the time that could carry out airstrikes if necessary.

Under heavy fire, U.S. troops called in French fighter jets for air support, but the firefight was at such close quarters that the planes could not engage and were instead left circling overhead.

U.S. officials have said French aircraft were overhead within 30 minutes.

The U.S. military’s Africa Command said the soldiers were in the area to establish relations with local leaders and deemed it unlikely that they would meet resistance.

A diplomat with knowledge of the incident said French officials were frustrated by the U.S. troops’ actions, because they had acted on only limited intelligence and without contingency plans in place.

U.S. forces do not have a direct combat mission in Niger, and instead provide assistance to its army that includes intelligence, surveillance and reconnaissance in their efforts to target violent extremist organizations.